Continuous casting steel plate for porcelain enameling excellent in formability resistance to occurrence of bubble or black point, and adhesion with porcelain enamel

ABSTRACT

A vitreous enamel steel sheet produced by a continuous casting, excellent in formability, bubbling and black spot resistance and adhesiveness, which comprises, in terms of mass %, 0.002% or less of C, 0.05 to 0.2% of Mn, 0.01% or less of Si, 0.004% or less of N, 0.015 to 0.05% of 0, less than 0.01% of P, 0.025% or less of S, 0.02 to 0.04% of Cu, 0.03 to 0.05% of Nb, and 0.03 to 0.07% of V, satisfying the inequality: 0.2&gt;Mn (%)−2.0·O (%)+0.8·V (%)+0.5·Nb (%), and the balance consisting of Fe and unavoidable impurities, and a method for producing the same.

FIELD OF ART

This invention relates to a vitreous enamel steel sheet excellent inenameling properties (bubbling and black spot resistance andadhesiveness) and formability and to a method of producing the same, andis characterized especially by producing the same through continuouscasting.

BACKGROUND ART

Vitreous enamel steel sheets have conventionally been produced by ingotcasting into capped steel or rimmed steel, primary rolling, hot rolling,cold rolling and, then, decreasing carbon and nitrogen concentrations toseveral tens of ppm or less through decarbonization by open coilannealing and, in addition, denitrification annealing. The vitreousenamel steel sheets produced following these process steps have,however, a problem of a high manufacturing cost because the ingotcasting and primary rolling process is included in the manufacturingsteps and the decarbonization/denitrification annealing is required.Another problem is that they are not applicable to final productsrequiring intensive deep drawing work.

Facing these problems, some vitreous enamel steel sheets produced bycontinuous casting have been proposed for the purpose of cuttingproduction costs. For example, vitreous enamel steel sheets produced bycontinuous casting of high oxygen steels, such as the one disclosed inJapanese Examined Patent Publication No. S57-49089, have excellentenameling properties. However, they still have problems that they cannotbe used for deep drawing applications due to their poor formability andthat they still require decarbonization annealing ordecarbonization/denitrification annealing by the open coil method inrelation to aging properties.

As a means to overcome the above problems, Japanese Unexamined PatentPublication No. S59-190331 discloses an ultra-low carbon, ultra-lownitrogen vitreous enamel steel sheet produced by continuous casting. Theproposed technology consists of controlling concentrations of C and N ofa high oxygen steel to an extremely low level of C<30 ppm or C+N<30 ppmin steelmaking processes. The reduction of the C and N concentrations insteel to such a low level, however, inevitably incurs an increase insteelmaking costs and, what is more, even if C+N<30 ppm is achieved,perfectly non-aging steel sheets cannot be obtained.

Some methods to achieve deep drawability and non-aging propertiesthrough the addition of Ti or Ti+Rem have been disclosed ascountermeasures against the above problem. The technologies disclosed inJapanese Unexamined Patent Publications No. S51-32417 and No. S52-128822are such examples. The former is a method to enhance deep drawabilityand obtain non-aging properties by eliminating C and N in solid solutionthrough addition of Ti+Rem to an ultra-low carbon steel and, inparallel, to improve fish scale resistance by forming (Ti, Rem)S bymeans of S added in a large quantity. By this method, good vitreousenameling properties can be obtained in two-coat enameling in which astrongly adhesive ground coat is used, but bubbling occurs in one-coatenameling and fish scale resistance is insufficient. The method hasanother problem of a low product yield due to frequent surface defectscaused by the addition of Rem. The latter is a method to improve deepdrawability by means of a Ti-added ultra-low carbon steel andadhesiveness in two-coat enameling by Cu addition, but it has a problemin that it cannot be applied to steel sheets for one-coat enameling.

In the above situation, after a series of arduous researches, theinventors of the present invention submitted Japanese Patent ApplicationNo. H9-274932 regarding a vitreous enamel steel sheet excellent in fishscale resistance and deep drawability and a method to produce the same,proposing a high oxygen vitreous enamel steel sheet containing Nb and Vand produced by continuous casting. The proposed technology couldprovide a steel sheet excellent in deep drawability, but there was aproblem that its bubbling and black spot resistance was inferior tototally Al-free high oxygen steels.

DISCLOSURE OF THE INVENTION

The object of the present invention is to overcome the above problems ofconventional steel sheets for vitreous enamel use and to provide adeep-drawable and non-aging vitreous enamel steel sheet produced bycontinuous casting and excellent in bubbling and black spot resistanceand adhesiveness in one-coat enameling, and a method to produce thesame.

The present invention has been established as a result of wide-rangedstudies for the purpose of overcoming the shortcomings of theconventional steel sheets and their production methods. The findings{circle around (1)} to {circle around (6)} described below were reachedas a result of examinations of the influences of the following chemicalcomposition and production conditions on formability and enamelingproperties of vitreous enamel steel sheets.

Chemical composition (in mass %):

0.0005 to 0.0025% of C, 0.05 to 0.35% of Mn, 0.015 to 0.07% of O, 0.005to 0.06% of Nb, 0.03 to 0.07% of V, 0.05% or less of Cu, 0.05% or lessof Si, 0.005 to 0.025% of P, 0.035% or less of S, and 0.0015 to 0.0035%of N.

Production conditions:

A reheating temperature of 1,250 to 1,050° C., a hot rolling finishingtemperature of 750 to 950° C., a coiling temperature of 500 to 800° C.,a cold reduction ratio of 50% or more, and an annealing at 650 to 850°C. for 1 to 300 min.

Enameling properties:

Fish scale resistance, bubbling and black spot resistance, surfacedefects and adhesiveness were examined regarding steel sheets coatedwith an enamel film of 100 μm in thickness by a one-coat enameltreatment after a pickling and a Ni treatment.

{circle around (1)} The lower the amounts of C and oxygen, the betterthe deep drawability.

{circle around (2)} A deep drawability of r-value >2.0 is attained byreducing C content to 0.002% or less and adding Nb to 0.03% or more.

{circle around (3)} An aging index of 5 MPa or less is obtainedregardless of annealing conditions, when the following conditions aresatisfied: C<0.002%, V≧0.03%, and Nb≧0.03%.

{circle around (4)} A good enamel adhesiveness is obtained when thefollowing inequality is satisfied:

0.2>T. Mn−2.0(O)+0.8(V)+0.5(Nb).

{circle around (5)} Bubbling and black spots do not occur on enameledsteel sheets also when the following inequality is satisfied:

0.2>T. Mn−2.0(O)+0.8(V)+0.5(Nb).

{circle around (6)} Hydrogen permeation time, which has good correlationwith fish scale resistance, is influenced by the contents of oxygen, Mn,V and Nb, and the larger the contents of these elements, the longer thehydrogen permeation time.

The gist of the present invention, which was established based on theabove facts, is as follows:

(1) A vitreous enamel steel sheet produced by continuous casting,excellent in formability, bubbling and black spot resistance and enameladhesiveness, comprising, in mass %, 0.002% or less of C, 0.05 to 0.2%of Mn, 0.01% or less of Si, 0.004% or less of N, 0.015 to 0.05% of 0,below 0.01% of P, 0.025% or less of S, 0.02 to 0.04% of Cu, 0.03 to0.05% of Nb, and 0.03 to 0.07% of V, satisfying the inequality 0.2>Mn(%)−2.0·O (%)+0.8·V (%)+0.5·Nb (%), and the balance consisting of Fe andunavoidable impurities.

(2) A method to produce a vitreous enamel steel sheet produced bycontinuous casting, excellent in formability, bubbling and black spotresistance and enamel adhesiveness, characterized by hot rolling acontinuously cast slab, comprising, in mass %, 0.002% or less of C, 0.05to 0.2% of Mn, 0.01% or less of Si, 0.004% or less of N, 0.015 to 0.05%of O, below 0.01% of P, 0.025% or less of S, 0.02 to 0.04% of Cu, 0.03to 0.05% of Nb, and 0.03 to 0.07% of V, satisfying the inequality 0.2>Mn(%)−2.0·O (%)+0.8·V (%)+0.5·Nb (%), and the balance consisting of Fe andunavoidable impurities, at a finishing temperature of 800° C. or higherand a coiling temperature of 600 to 800° C., cold rolling the hot rolledstrip at a reduction ratio of 60% or more, and then annealing the coldrolled strip at a recrystallization temperature or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the influence of Mn on bubbling resistance ofNb—V steels.

FIG. 2 is a graph showing the influence of P on bubbling resistance ofNb—V steels.

FIG. 3 is a graph showing the relationship between the expressionMn−2.0·O+0.8·V+0.5·Nb and enamel adhesiveness based on theinvestigations of the one-coat enamel adhesiveness of steel sheets withdifferent chemical compositions.

FIG. 4 is a graph showing the relationship between the expressionMn−20·O+0.8·V+0.5·Nb and enamel surface properties based on theinvestigations of the enamel surface properties of steel sheets withdifferent chemical compositions.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail hereunder.

In the first place, the steel chemical composition will be explained indetail.

It has long been known that the lower the amount of C, the better theformability of steel sheets. In the present invention it is necessary tolower C content to 0.002% or less in order to achieve an aging index ofbelow 5 MPa and an r-value over 2.0. A more preferable C content rangeis 0.0015% or less. Although it is not necessary to specify a lowerlimit of C, a practical lower limit is 0.0005% since further reductionof C content results in increased steelmaking costs.

Si content has to be controlled to 0.01% or less since Si is detrimentalto enameling properties. A more preferable Si content range forobtaining good enameling properties is 0.005% or less.

Mn is an important element to affect enameling properties in combinationwith addition of oxygen, V and Nb. Mn also prevents hot shortness causedby S during hot rolling, and 0.05% or more of Mn is required in thesteel according to the present invention which comprises oxygen. Whenthe Mn content is high, however, enamel adhesiveness is adverselyaffected and bubbling and black spots are likely to occur. For thisreason, the upper limit of Mn content is defined to be 0.2% from FIG. 1based on the relationship between Mn content and the occurrence ofbubbling under the same condition as in the case of FIG. 4. A morepreferable upper limit of Mn content is 0.17%.

Oxygen has direct effects on fish scale resistance and formability. Italso affects adhesiveness and bubbling and black spot resistance incombination with the contents of Mn, Nb and V. At least 0.015% of oxygenis necessary for these effects to show but, when its content is high,its high concentration directly deteriorates formability and, besides,deteriorates formability and aging properties indirectly by hinderingthe effects of Nb and V addition. Hence, its upper limit is set at0.05%.

N is an interstitial solid solution element like C. When its contentexceeds 0.004%, however, formability is lowered even with the additionof Nb and/or V, and the production of non-aging steel sheets becomesimpossible. The upper limit of N content is thus specified to be 0.004%.Although it is not necessary to specify a lower limit of N, a practicallower limit is 0.001% since reduction of N content to 0.001% or less iscostly by the present steelmaking technologies.

P accelerates pickling speed in a pretreatment process for enamelingwhen its content is 0.01% or more, resulting in an increase in smuts,which give rise to bubbling and black spots. For this reason, P contentis limited to below 0.01% in the present invention, based on therelationship between the amount of P and bubbling resistance seen inFIG. 2, under the same condition as that of FIG. 4. A significantimprovement of the bubbling and black spots can be achieved by reducingthe P content to below 0.01%. The mechanism of this improvement is notclear, but it is supposed as follows: P tends to precipitate at grainboundaries and forms FeP compounds, which give birth to smuts, etc.during pickling and thus tends to cause the bubbling and black spots.For this reason, it is necessary to control the P content to below 0.01%for suppressing the formation of the FeP compounds. When the amount of Pis 0.01% or more, the concentration of P at grain boundaries increases,and the FeP compounds form easily. The compounds cause formation oflocal batteries during the pickling process due to potential differencebetween the grain boundary and matrix, resulting in locally acceleratedpickling speed, and smuts occur in a great quantity during the course ofthis phenomenon. In order to suppress the occurrence of smuts duringpickling, hence, it is extremely important to minimize the formation ofthe FeP compounds by preventing P from precipitating at grainboundaries.

S increases smuts, during pickling to pretreat the steel sheets prior toenameling, giving rise to the bubbling and black spots, thus it isnecessary to keep its content at 0.025% or less.

V is an important element in the present invention. V immobilizes N toprevent N-induced deterioration of deep drawability and that of pressformability caused by aging-induced decrease in elongation. A part of Vadded to steel forms oxides combining with oxygen in the steel andprevents fish scales from occurring. It also has an indirect effect toenhance formability by lowering the amount of oxygen required forsuppressing the occurrence of fish scales. For these reasons, the lowerlimit of V content is set at 0.03%. An addition of V in a great quantitydeteriorates enamel adhesiveness and bubbling and black spot resistanceand, hence, its upper limit is set at 0.07%.

Nb is another important element in the present invention. It improvesdeep drawability of steel sheets and renders them non-aging byimmobilizing C. Nb added to steel also forms oxides with oxygen in thesteel and prevents fish scales from occurring. It also has an indirecteffect to enhance formability by lowering the amount of oxygen requiredfor suppressing the occurrence of fish scales. In order to exhibit theseeffects, at least 0.03% of Nb is necessary. An addition of Nb in a greatquantity, however, deteriorates enamel adhesiveness and bubbling andblack spot resistance and, hence, its upper limit is set at 0.05%.

Cu is well known to lower pickling speed, during the pickling topretreat steel sheets prior to enameling. In the present invention, atleast 0.02% of Cu is required to fully enjoy the above effect. When thepickling speed lowering effect is too strong, however, enameladhesiveness is lowered when the pickling time is short since the steelaccording to the present invention contains extremely small amounts of Cand N in solid solution due to Nb and V addition. Thus, the upper limitof Cu is set at 0.04%.

In order to obtain good enamel adhesiveness and bubbling and black spotresistance, contents of Nb, V, Mn and O have to satisfy the inequality0.2>Mn (%)−2.0·O (%)+0.8·V (%)+0.5·Nb (%). The relationship of theinequality has been defined based on the test results shown in FIGS. 3and 4 obtained through in-depth examinations of the relationship betweensteel chemistry and enameling properties.

FIG. 3 shows the relationship between adhesiveness and steel chemistry.Here, the adhesiveness was measured in the following manner: steelsheets of 0.7 mm in thickness were pickled in 15% H₂SO₄ at 75° C. for 3min., Ni-treated in 2% NiSO₄ at 70° C. for 3 min. while the pH changedfrom 2.5 to 3.5, glazed with a glaze for one-coat enameling in thethickness of 100 μm, baked at 840° C. for 3 min., then hit by a 2.0-kgspherical-headed weight falling from a height of 1 m, and the area ofthe portion deformed by the impact where the enamel has not peeled offwas measured with 169 probing needles. The adhesiveness value in thefigure is a mean value of all the specimens of a group having the samevalue of the expression Mn−2.0·O+0.8·V+0.5·Nb.

FIG. 4 shows the relationship between the occurrence of bubbling andblack spots and steel chemistry. Here, the occurrence of bubbling andblack spots was measured in the following manner: steel sheets werepickled in 15% H₂SO₄ at 75° C. for 20 min, Ni-treated, enameled, baked,etc. under the same conditions as in FIG. 3, and the occurrence ofbubbling and black spots was evaluated by visual inspection of surfacesof the enameled sheets. The symbols mean as follows: ⊚ no occurrence ofbubbling and black spots, ∘ limited occurrence, and X high occurrence.

Since other unavoidable impurities adversely affect steel materialproperties and enameling properties, it is desirable to minimize theiramounts.

Next, the production method will be described hereunder. Steel slabsaccording to the present invention are produced through continuouscasting and the cast slabs are subsequently hot rolled. Since theadvantages of the present invention are not affected by reheatingtemperature, a commonly practiced reheating temperature range of 1,050to 1,250° C. is applicable. Any hot rolling finishing temperature of800° C. or higher is acceptable but, in consideration of hot rollingoperability, a temperature equal to Ar₃ point or higher is preferable.As for coiling temperature, in order to obtain good formability, it ispreferable to apply cooling by 50° C. or more immediately after thefinal rolling pass.

A cold reduction ratio of 60% or more is required for obtaining steelsheets having good deep drawability. When good deep drawability isespecially required, a cold reduction ratio of 75% or more ispreferable.

As for annealing, advantages of the present invention can be enjoyedeither through box annealing or continuous annealing as far as arecrystallization temperature or above it is attained. Continuousannealing is preferable especially for obtaining excellent deepdrawability and good enameling properties, which are characteristic ofthe present invention. Since the steel according to the presentinvention is characterized in that its recrystallization is completed at650° C. even within a short annealing time, it is not necessary toanneal at an especially high temperature. A generally suitabletemperature range is from 650 to 750° C. for box annealing and from 700to 800° C. for continuous annealing.

As explained hereinbefore, the steel sheets having the chemicalcomposition of the present invention and produced under themanufacturing conditions according to the present invention are vitreousenamel steel sheets excellent in press formability equal or superior toconventional decarbonized capped steels, not prone to the occurrence ofbubbling and black spot defects at direct one-coat enameling, andexcellent in enamel adhesiveness, even when they are produced fromcontinuously cast slabs. In applications such as bathtubs and kettles towhich an enameling process other than the direct one-coat enameling isemployed, the advantages of the invented steel sheet do not change atall and can be equally enjoyed. Note that the conventional ingot-makingand primary rolling method is also applicable insofar as thecharacteristics of the present invention are maintained.

EXAMPLE

Continuously cast slabs having the chemical compositions shown in Table1 were hot rolled, cold rolled and annealed under the conditionsspecified in Table 2, then skin pass rolled at a reduction ratio of1.0%, and the mechanical properties and the enameling properties ofsteel sheets thus produced were examined. The results are shown also inTable 2.

The mechanical properties were investigated regarding tensile strength,r-value and aging index, using JIS No. 5 test pieces prepared from theannealed and 1.0% skin pass rolled steel sheets. The aging index isshown in the form of stress difference between before and after an agingat 100° C. for 60 min. in the steel sheets subjected to a pre-strain of10%.

The enameling properties were evaluated after the process steps listedin Table 3. Among the enameling property items, the surface propertywith respect to bubbling and black spots was tested under the conditionof a long pickling time of 20 min. and the evaluation result isindicated as follows: ⊚ no occurrence of bubbling and black spots, ∘limited occurrence, and X much occurrence.

The enamel adhesiveness was evaluated under the condition of a shortpickling time of 3 min. Because the commonly used P.E.I. adhesivenesstest method (ASTM C313-59) is incapable of detecting small difference inthe enamel adhesiveness, it was evaluated by hitting test pieces with a2.0-kg spherical-headed weight falling from a height of 1 m andmeasuring the ratio of the area where the enamel has not peeled off tothe whole deformed area with 169 probing needles.

Fish scale resistance was evaluated through the following acceleratedfish scale test: three steel sheets were pretreated through a 3-min.pickling without Ni immersion, glazed with a glaze for direct one-coatenameling, dried, baked at 850° C. for 3 min. in a baking furnace havinga dew point of 50° C., and then held for 10 hours in a constanttemperature oven of 160° C. The occurrence of fish scales was visuallyjudged and the result is shown in Table 2.

As is clear from the results shown in Table 2, the steel sheetsaccording to the present invention show good r-value and El, and arevitreous enamel steel sheets excellent in aging resistance and enamelingproperties. The steel sheets shown as comparative examples provedinferior in one or both of material properties and enameling properties.This means that a steel sheet excellent in material and enamelingproperties cannot be produced unless chemical composition and the closerelationship among component elements are kept within the rangesspecified in the present invention.

TABLE 1 Chemical composition (wt %) Mn-2 × 0 + Steel C Si Mn P S N O CuNb V 0.8 V + 0.5 Nb Remarks A 0.0010 0.003 0.19 0.009 0.015 0.0021 0.0310.029 0.038 0.050 0.197 Examples of B 0.0015 0.001 0.18 0.008 0.0120.0016 0.045 0.031 0.045 0.060 0.183 present C 0.0012 0.003 0.15 0.0070.011 0.0020 0.028 0.026 0.033 0.040 0.143 invention D 0.0007 0.003 0.190.008 0.012 0.0025 0.032 0.030 0.039 0.055 0.189 E 0.0010 0.004 0.190.007 0.017 0.0022 0.032 0.029 0.040 0.051 0.182 F 0.0006 0.002 0.180.009 0.016 0.0027 0.033 0.025 0.039 0.060 0.182 G 0.0009 0.005 0.140.008 0.013 0.0023 0.045 0.026 0.043 0.052 0.113 H 0.0035 0.008 0.220.015 0.015 0.0030 0.050 0.030 0.035 0.040 0.180 Comparative I 0.00200.008 0.40 0.018 0.018 0.0035 0.033 0.030 0.035 0.042 0.381 examples J0.0020 0.005 0.30 0.015 0.016 0.0050 0.050 0.031 0.036 0.035 0.246 K0.0019 0.012 0.25 0.014 0.016 0.0035 0.010 0.033 0.030 0.035 0.273 L0.0019 0.012 0.29 0.016 0.014 0.0030 0.080 0.029 0.030 0.035 0.193 M0.0015 0.005 0.29 0.015 0.016 0.0028 0.045 0.030 0.022 0.035 0.243 N0.0019 0.005 0.29 0.017 0.020 0.0027 0.048 0.029 0.032 0.020 0.226 O0.0010 0.003 0.29 0.015 0.018 0.0025 0.022 0.030 0.031 0.030 0.286 P0.0015 0.001 0.21 0.014 0.012 0.0016 0.045 0.031 0.045 0.060 0.183

TABLE 2 Hot rolling conditions Cold Enameling properties ReheatingFinishing Coiling reduc- Fish Surface tempera- tempera- tempera- tionAnnealing Mechanical properties scale Adhesive- properties, ture tureture ratio (° C. × YP TS El r- Al resis- ness bubbling and Re- Steel (°C.) (° C.) (° C.) (%) min.) (MPa) (MPa) (%) value (MPa) tance (%) blackspots marks A 1200 910 750 78 750 × 1  156 290 52 2.2 0.0 ⊚ 100 ⊚ Exam-B 1150 890 700 80 800 × 1  149 290 52 2.3 0.0 ⊚ 100 ⊚ ples C 1100 900750 80 750 × 1  160 300 52 2.2 0.0 ⊚ 100 ⊚ of D 1200 890 700 80 750 × 1 142 289 54 2.4 0.0 ⊚ 100 ⊚ present E 1050 915 745 80 775 × 1  170 295 522.5 0.0 ⊚ 100 ⊚ inven- F 1250 920 750 78 700 × 300 165 280 53 2.6 0.0 ⊚100 ⊚ tion G 1200 900 700 85 680 × 300 170 290 53 2.3 0.0 ⊚ 100 ⊚ H 1200900 700 75 750 × 1  190 310 48 1.6 35.0 ⊚ 100 ◯ Com- I 1200 890 650 80750 × 1  180 300 20 2.0 3.0 ⊚ 70 X para- J 1200 890 700 75 750 × 1  195312 47 1.7 38.0 ⊚ 88 X tive K 1250 900 650 80 750 × 1  160 285 53 2.40.0 X 80 X exam- L 1250 915 700 80 700 × 300 180 295 50 1.8 41.2 ⊚ 100 Xples M 1200 900 700 78 750 × 1  221 332 46 1.7 36.5 ◯ 85 X N 1200 895650 78 750 × 1  215 329 47 1.6 39.1 ◯ 85 X O 1150 880 600 78 750 × 1 175 288 52 2.2 3.2 ⊚ 80 X P 1200 890 500 75 750 × 1  180 325 46 1.7 28.0⊚ 100 ◯ ⊚: No occurrence ◯: Limited occurrence X: Much occurrence

TABLE 3 Process steps Description 1 Degreasing Alkaline degreasing 2Washing with hot water 3 Washing with cold water 4 Pickling 15% H₂SO₄,75° C. × 3- or 20-min. immersion 5 Washing with cold water 6 Nitreatment 2% NiSO₄, 70° C. × 3-min. immersion 7 Washing with cold water8 Neutralization 2.0% Na₂SO₄, 75° C. × 5-min. immersion 9 Drying 10Glazing Direct one-coat glaze, 100 μm in thickness 11 Drying 160° C. ×10 min. 12 Baking 840° C. × 3 min.

Industrial Applicability

The vitreous enamel steel sheets according to the present invention havedeep drawability as good as or superior to that of conventional Ti-addedhighly press-formable steels, and satisfy all the bubbling and blackspot resistance, enamel adhesiveness and surface properties required ofvitreous enamel steel sheets. What is noteworthy is that the presentinvention overcomes bubbling and black spot defects, which constituted aproblem with the Ti-added steels, that it achieves surface propertiesequal or superior to those of decarbonized capped steels even whencontinuous casting is employed, and that it remarkably reduces slabmanufacturing costs. The present invention largely contributes also toreducing costs of annealing since it makes it possible to produce steelsheets excellent in press formability and aging resistance throughcontinuous annealing or box annealing, similar to high oxygen steelsconventionally produced through continuous casting, without requiringdecarbonization annealing or decarbonization/denitrification annealing.

What is claimed is:
 1. A vitreous enamel steel sheet produced bycontinuous casting, excellent in formability, bubbling and black spotresistance and enamel adhesiveness, comprising, in mass %, 0.002% orless of C, 0.05 to 0.2% of Mn, 0.01% or less of Si, 0.004% or less of N,0.015 to 0.05% of O, below 0.01% of P, 0.025% or less of S, 0.02 to0.04% of Cu, 0.03 to 0.05% of Nb, and 0.03 to 0.07% of V, satisfying theinequality 0.2>Mn (%)−2.0·O (%)+0.8·V (%)+0.5·Nb (%), and the balanceconsisting of Fe and unavoidable impurities.
 2. A method to produce avitreous enamel steel sheet produced by continuous casting, excellent informability, bubbling and black spot resistance and enamel adhesiveness,characterized by hot rolling a continuously cast slab, comprising, inmass %, 0.002% or less of C, 0.05 to 0.2% of Mn, 0.01% or less of Si,0.004% or less of N, 0.015 to 0.05% of O, below 0.01% of P, 0.025% orless of S, 0.02 to 0.04% of Cu, 0.03 to 0.05% of Nb, and 0.03 to 0.07%of V, satisfying the inequality 0.2>Mn (%)−2.0·O (%)+0.8·V (%)+0.5·Nb(%), and the balance consisting of Fe and unavoidable impurities, at afinishing temperature of 800° C. or higher and a coiling temperature of600 to 800° C., cold rolling the hot rolled strip at a reduction ratioof 60% or more, and then annealing the cold rolled strip at arecrystallization temperature or higher.